This invention relates to pulse width modulated control system for a d.c. motor.
Control of motors by pulse width modulated signals is well known as described, for example, in U.S. Pat. No. 3,213,343 issued to Sheheen on Oct. 19, 1965. Such controls provide a superior way to smoothly accelerate and decelerate a d.c. motor under various circumstances. This is particularly advantageous when the motor is applied to a device such as an exercise treadmill or bed. It is particularly desireable to ensure operator comfort under a variety of possible circumstances such as fluctuating in the a.c. current input or failed components. Additionally, it is desireable to protect the other components in the circuit due to surges in current or voltage experienced under abnormal operating conditions of the product being operated by the motor.
Another important consideration in the operation of any pulse width modulation control system is the ability to transfer maximum power to the load through a power switch. Four factors that contribute to system losses are:
(1) input or driving power losses; PA2 (2) saturation or static losses when the system is on; PA2 (3) switching or dynamic losses that result from the transition times when the device is turned on and off; and PA2 (4) off losses due to the product of leakage current and power supply voltage.
Since input power losses can be substantial for various semiconductors, the metal oxide -silicon field effect transistor or MOSFET has found increasing acceptance as a power switch device since it has an extremely high static input impedance which allows it to turn on with significantly less input power.
Still another desireable feature in any control system is to ensure that the bridge rectification components such as silicon controlled rectifiers are triggered accurately with appropriate sensitivity. It is important that the stability of the triggering device such as a programmable unijunction transistor be observed carefully. The above and other features are addressed by the circuit described below.